A precision TCXO like this nearly eliminates frequency drift. The stock crystal on the dongles isn’t that accurate, which can cause your tuned frequency to migrate around a bit. That’s not good for reception, especially if you’re trying to receive digital data or precisely time align multiple dongles like in a trunking or diversity application. RTL-SDR.com made these great recordings of the stability to illustrate the point:

On the left with a stock crystal, the dongle’s tuning wobbles around the center frequency. On the right with the TCXO, it’s dead stable.